The present invention relates to semipermeable mosaic polymer membranes of asymmetric structure and with a macroscopic distribution of the mosaic-forming anionic and cationic charges (sites). These membranes have good permeability for electrolytes, such as salts of mono- or polyvalent inorganic acids, while retaining low molecular weight organic solutes.
The separation of mono-, di- or polyvalent salts, such as sodium chloride, or sodium sulfate or sodium triphosphate, from low molecular weight (MW&lt;1000) organic compounds in (aqueous) solutions, via membranes is an important industrial separation problem which has not been economically solved. Membranes have been shown to offer an economical solution to many separation problems because of their ability to concentrate without a phase change, and to separate different solutes. The traditional membrane process of reverse osmosis (RO) rejects all salts and organics. The relatively newer membranes of selective reverse osmosis cannot efficiently achieve the above separations even though they are designed to pass salt and retain the organic solutes. The mode of separation in selective RO is based on size and electrostatic discrimination, and the proper choice of materials has not been found to give e.g. a sulfate passage. However, membrane structures containing separated macroscopic domains (0.05 to 100 microns) of anionic and cationic ion exchange materials connecting the opposite faces of the rejecting layer (called a charged mosaic membrane) have a built-in salt transport mechanism. They have been postulated and shown to give separation between organic solutes and salts. Under a pressure gradient the membranes preferential transport salt across the mosaic while retaining the organic solute [H. Kowatoh et al., Macro-molecules 21, 625-628, 1988].
Mosaic membranes have also been shown to give high water flux, while, at the same time, giving a permeate enriched in salt (F.B. Leitz, J. Shore, Office of Saline Water, Res. Developm. Program Report No. 775 (1972)].
Mosaic membranes are membranes with a macroscopic distribution of cationic and anionic sites. Typically, though not exclusively, they are arranged as particles, such as cationic and/or anionic particles distributed in a neutral matrix, or particles of one charge distributed in a matrix of the other charge. In this case, particles may be defined as regular or irregular approaching such shapes as spheres, multisided, fibers, cones, and others.
The different approaches to achieve the structures of mosaic membranes comprise such methods as the introduction of preformed particles in a matrix via resin suspension in a casting solution of the matrix, block or random copolymerisations, or phase separation in a common solvent (material incompatibility).
Mosaic membranes have not yet become commercially important in separation processes because of the difficulty in upscaling a reproduceable process for making mosaics. Of all the above approaches, one of the simplest with a good upscaling and fabrication potential is the said material incompatibility, or phase separation in a common solvent. However, it is also the most difficult to make without imperfections which destroy membrane properties.
Further, the known mosaic membranes do not have high enough rejection to low (less than 400) molecular weight solutes, or if they have high enough rejection their water fluxes are too low.